But a key question has remained unanswered: Is it better to get the stem cells from a donor's blood or from bone marrow?

Now, a new study evaluates the pros and cons of harvesting stem cells from bone marrow rather than blood and suggests there are benefits to both approaches, but no survival differences between the two methods. The research was published Oct. 18 in the New England Journal of Medicine.

Over the past 10 years, 75 percent of stem cell transplants from unrelated adult donors have used peripheral blood stem cells rather than those harvested from bone marrow, according to study background information.

Some studies have suggested that using peripheral blood cells rather than bone marrow was associated with more severe GVHD. Other research has found that some people with transplants from peripheral blood stem cells had a lower relapse rate and improved survival.

Bone marrow offers the same chances of survival as does peripheral blood but tends to be associated with more severe side effects of treatment, explained study author Dr. Claudio Anasetti, a professor of medicine at the University of South Florida.

"With bone marrow, you have the same survival, but less long-term morbidity," Anasetti said.

Anasetti said the research shows that both approaches are acceptable, but "it's not a one-choice-for-all situation."

In some diseases, the blood-forming stem cells in the bone marrow stop working effectively and fail to generate enough new blood cells, or the right types of cells.

In other cases, people who have had had high doses of radiation or chemotherapy to destroy life-threatening cancer cells, the treatment can stop their bone marrow's ability to make blood cells.

Stem cell transplants allow physicians to replace the body's source of blood cells after the bone marrow and its stem cells have been destroyed.

GVHD is a common side effect among those who receive cells from an unrelated donor. Transplanted cells attack the patient's tissue, causing a range of issues, from skin rashes and diarrhea to serious liver problems. Chronic GVHD typically occurs between three months to three years after the transplant.

Dr. Fred Appelbaum, director of the clinical research division at the Fred Hutchinson Cancer Research Center in Seattle, argued in an accompanying editorial that adult stem cells harvested from bone marrow rather than blood should be the norm.

"If the donor and recipient are not identical twins, the graft attacks its new host. We give immune suppression to reduce this response," Appelbaum said in an interview. "But for those who get peripheral blood, there's a higher incidence of a severe, chronic immune response that can greatly diminish a person's quality of life."

For the last decade, peripheral blood has grown in popularity as a source of adult stem cells for transplant because they are easier to obtain from the donor and can be stimulated to rapidly grow even before harvesting. They also tend to establish themselves quickly inside the recipient's body.

Would more emphasis on doing bone marrow transplant rather than blood donation potentially reduce the donor pool? Appelbaum said while bone marrow donation sounds much more arduous than does blood donation, in reality they both involve a comparable level of commitment.

Donating stem cells from bone marrow usually involves receiving general anesthesia for removal of the marrow from the hip bone by needle. The process of donating peripheral blood stem cells includes taking medication -- granulocyte colony-stimulating factor or GCSF -- for four or five days, which typically causes five days of slowly building bone and muscle pain, explained Appelbaum.

Then these donors undergo a process called apheresis, a four-to-six-hour process in which blood is removed through a large vein in the art, neck, chest or groin and put through a machine to pull out the stem cells.

No matter how the stem cells are harvested, the recipient is first treated with high-dose anticancer drugs and/or radiation, and then receives the donor cells through an intravenous infusion.

Appelbaum said about 5,500 unrelated donor transplants were performed in the United States last year. More than 20 million unrelated donors are typed and listed in registries in North and South America, Europe and Asia.

The large, multicenter, randomized trial compared survival rates and side effects of treating people with either hematopoietic (blood-forming) adult stem cells obtained from bone marrow or adult stem cells derived from circulating blood.

Between March 2004 and September 2009, the researchers enrolled 551 patients at 48 centers. Patients were randomly assigned to receive either peripheral blood stem cell or to bone marrow transplantation, based partially on their disease risk and transplantation center. The average follow-up of surviving patients was at 36 months.

In addition to showing no survival difference between transplant recipients who received bone marrow or peripheral stem cells, the study showed no difference in relapse rates, mortality unrelated to relapse or rate of acute GVHD.

Those who received peripheral blood transplants had better engraftment -- the process of transplanted cells producing new cells -- and the bone marrow transplant patients experienced less extensive chronic GVHD.

Appelbaum said that the results of the study should change practice. For most unrelated donor transplants, bone marrow rather than peripheral blood should be the first choice, he argued. But he's unsure whether it will.

"The benefits of peripheral blood are seen early, under the watchful eyes of the transplant physician, while the deleterious effects occur late, often after the patient has left the transplant center," his editorial concluded.